Instruments in which an illuminated display is useful in include clocks, hand held computer games, mobile phones, and any other instrument which is capable of displaying information electronically. Instruments for displaying information on a vehicle display are usually located on the dashboard of an automotive vehicle, and require some form of illumination to allow the driver to read the information when it is dark. An instrument usually comprises an information bearing surface in a housing having a transparent cover for protection of the instrument. The information bearing surface usually has symbols indicating a scale such as graphics, numbers, or a dial, and a pointer for indicating a point within the scale, or may comprise a liquid crystal display suitable for displaying numeric information. The information bearing surface may also include features such as rings, which define the perimeter of a particular instrument or gauge. The instrument is generally mounted on a printed circuit board (PCB) which is connected to sensors (directly of indirectly via a network) which monitor certain engine conditions such as speed, temperature and fuel level, for example. Conventionally instruments are mounted together to form an instrument cluster. As customer requirements become more sophisticated, it becomes important to provide lighting for vehicle instruments which is both distinctive and cost effective to produce.
Generally, one of two types of illumination is used, either front lighting or back lighting. Front lighting refers to illumination from the viewing side of the information bearing surface which reflects off the information bearing surface. The light need not be perpendicular to the information bearing surface, hence this term includes side lighting, where the light is reflected from the information bearing surface. The information is printed so that the light reflecting from the symbols is in contrast to the light reflecting from a background, for example the symbols may be black and the background may be white or the symbols may be white and the background may be black. When the symbols are lighter than the background the illumination is known as negative mode, and when the symbols are darker than the background the instrument is described as positive mode.
Back lighting refers to illumination of the information bearing surface from the side opposite to the viewing side of the instrument. In this case the information surface contains opaque or semi opaque portions so that the light passing through the information bearing surface renders the symbols visible. Backlighting of vehicle instrument panels has become more common as it is currently considered to provide a better overall appearance for the vehicle instrument panel because it is not required to mount lighting components in front of the instrument.
Front or back lighting may be achieved either by providing illumination directly from a light source (which may include the use of reflective surfaces), or indirectly from a light source which is coupled to a light guide which directs the light from the light source to the required illumination point.
A problem with known instruments which have an associated light source is that, while front lighting allows a simpler design for the information bearing surface and any associated pointers, any cost savings resulting from these simplified designs are negated by the extra costs required for flexible connectors from the PCB to the light source or complex light guides, and for more complex assembly. Front lighting from a point source also often produces shadows. Back lighting on the other hand usually involves the use of a plurality of light sources, and a more complex arrangement is required to illuminate the pointer. Furthermore, applying symbols to the information bearing surface is more complex in the back lighting case because poor distribution of the illumination can cause portions of the instrument to have better illumination than others. This is compensated for by providing different thickness of opaque or semi opaque portions in some areas than in others. However an undesirable effect commonly referred to as “color shift” occurs when using a compensation method and that is particularly noticeable with white color illumination. The color shift is caused by the difference in the coefficient of transmission of light in distinct areas of the information bearing surface.
A further problem with backlighting is that as light sources develop and more light emission becomes possible, particularly with light emitting diodes (LEDs), distribution of that light in an effective manner becomes more difficult.
It is known to use an optical fiber for illumination of instrument panels, for example DE19940849 discloses an optical fiber for illuminating an instrument panel which has a section for radial emission of light.
It is known to use a light conducting member around the edge of a dial for the purpose of illuminating an instrument. For example JP2006194613 discloses arc shaped light guide plates; US2006066250 discloses a glass panel and curved tube light guide made of lighting sensitive material. GB482595 discloses a solid rod-like light conducting member around the edge of a dial. The member may be covered by opaque material except for two slits on the side facing the dial and may be placed behind the cover.
It is known to provide cuts or discontinuities to allow light to escape from an optical fiber. For example, WO8400800 discloses a flexible transmitting guide which may have cuts at intervals along its surface; JP57063469 describes a device for illuminating a watch which has optical fiber with plurality of cuts for emitting light; EP0879991 discloses an illuminating system having a light guide member with slits.
Most conventional instrument clusters are illuminated using several light sources arranged into a circular array of LEDs beneath the graphical information area. A small reduction in the number of LEDs used can be achieved in combination with a balancing technique. Such balancing consists of darkening any bright spots to the level of the lowest brightness spots. However, as mentioned previously, this method induces an undesirable color shift that occurs when a polychromatic source is employed such as white light due to the difference in the spectrum of absorption of light induced by the additional painting or darkening layers.
Another method is to use indirect illumination as disclosed in WO9926809. Such indirect illumination employs few LEDs that are located near the center of the gauge and the emitted light is reflected and diffused in the outward direction by a diffusing deflector. The drawback of this method is that no space is available near the central area of the gauge where it is commonly desirable to have warning lights around the pointer.
To reduce the number of LEDs used and the cost, light guides have increasingly been used. A light guide solution, as disclosed in JP2001281008 uses a small number of LEDs in comparison with direct or indirect illumination, but the LEDs required occupy most of the space on the PCB around a mount for a pointer where warning lights are commonly desired.
European Patent EP0498451 discloses a light guide device for evenly backlighting vehicle instruments where a circular light transmitting rod having a reflection portion formed on a boundary surface is disposed behind a graphical display. The reflection boundary surface is made by applying a diffusing painting layer or by introducing scattering particles, incurring additional tooling costs. Furthermore, the light source is placed at one or both ends of the circular light guide where it is highly desirable to have warning lights or other display features.
There is a need to develop efficient illumination devices that uses a minimum number of LEDs as well as providing uniform illumination and package compactness for use in vehicle instrument clusters.